1. Field of the Invention
The present invention relates to a liquid crystal lens apparatus comprising a liquid crystal cell layer including transparent substrates and liquid crystal filled therebetween and electrodes on the liquid crystal cell layer, changing the refractive index of the liquid crystal by controlling a voltage applied between the electrodes, thereby adjusting the focal length.
2. Description of the Related Arts
Liquid crystal lenses used as varifocal lenses in focusing mechanisms in optical disk devices, cameras, and other known optical devices have been proposed.
One example is a liquid crystal lens including two transparent substrates facing each other in the form of a lens as a whole and liquid crystal sealed therebetween and being capable of adjusting the refractive index of the liquid crystal by changing a voltage applied to transparent electrodes disposed on the transparent substrates (see, for example, Japanese Unexamined Patent Application Publication No. S63-206721).
Another example is a flat liquid crystal lens including two flat transparent substrates, transparent electrodes disposed on the opposing inner surfaces of the transparent substrates, and liquid crystal sealed in a flat gap between the substrates, at least one of the transparent electrodes including a plurality of concentric electrodes elements disposed concentrically about the optical axis of the liquid crystal lens (see, for example, Japanese Unexamined Patent Application Publication No. H5-053089). In the flat liquid crystal lens, a voltage applied to each transparent electrode elements of the transparent electrodes is changed to generate a radially varying voltage distribution, and thereby change the refractive index of the liquid crystal.
Although it is not a technique regarding a liquid crystal lens, a technique for heating a liquid crystal panel used in a display by using an electric heating element on the liquid crystal panel to aim to avoid degradation of operating characteristics at low temperatures has been proposed (see, for example, Japanese Unexamined Patent Application Publication No. 2004-170852).
The inventors of the present invention have proposed a technique for forming an electric heating element on a glass substrate of a liquid crystal panel (see, for example, Japanese Unexamined Patent Application Publication No. H11-1194358).
In order to apply a liquid crystal lens to zooming function or focusing function of a camera, it is necessary to increase the variable range of a focal length. To this end, the refractive index is required to be much changeable.
For largely changing the refractive index of a flat liquid crystal lens, it is necessary to increase the refractive index anisotropy, Δn, of a liquid crystal material used in the liquid crystal lens or to increase the distance, d, between the substrates. However, there is a limit to the magnitude of the refractive index anisotropy. The refractive index anisotropy Δn is typically 0.4 or less. Therefore, it is necessary to increase the distance d between the substrates in order to obtain a desired change in the refractive index. In this case, the response speed of the liquid crystal lens to a driving voltage applied thereto decreases in proportion to the square of the distance d between the substrates.
For a liquid crystal lens used as a focusing mechanism of a camera, the response time is required to be no more than 2 seconds.
FIG. 24 is a graph showing the response time characteristics LC1 and LC2 of liquid crystal occurring when a driving voltage is applied across the liquid crystal in the case where the distance d between substrates sandwiching the liquid crystal is 10 μm and 25 μm. As is shown in FIG. 24, the response time characteristic LC1 for 10 μm of a distance d between the substrates is not longer than 2000 ms even at −10° C., whereas the response time characteristic LC2 for 25 μm of a distance d between the substrates is longer than 2000 ms even at 5° C.
Therefore, a liquid crystal cell having an increased distance d between the substrates for a liquid crystal lens is unable to function as a lens of a camera at low temperatures.
As mentioned the above, when a liquid crystal lens is functioned to control a focus of a camera, there arises a problem that an increased distance d between the substrates necessary for an increased focusing range makes the response time longer. An effective solution to this problem has not been found in existing liquid crystal lenses, in particular, those using low-voltage power supply such as battery power supply (e.g., electronic cameras). Therefore, a camera's focusing mechanism utilizing a liquid crystal lens is not yet in actual use.
The assignee of the present invention has proposed a liquid crystal lens using a heating system with an electric heating element and having an increased focusing range and an improved response speed in U.S. application Ser. No. 11/385,494. In the liquid crystal lens, liquid crystal is heated by heating elements, for example, in a shape of ring, disposed around electrodes on both side surfaces of a liquid crystal layer.
However, according to a further research conducted by the inventors of the present invention, in the case where a metal film having a diameter of approximately 5 mm, a width of approximately 200 μm, and a sheet resistance of 3Ω was used as an electric heating element, when a voltage of 3V, which was typically used in an electronic camera, was applied, a current of 60 mA was passed, consuming a power of 180 mW, and the temperature of the heating element rose only 20° C. This result shows that insufficient temperature compensation may occur in such a case. Therefore, in order to increase the overall liquid crystal lens temperature to a high temperature required for sufficient temperature compensation, it is necessary to transmit local high-temperature heat to the entire liquid crystal cell and to minimize idle heat dissipation to the outside.